The invention relates to an apparatus for the classification or separation of solid and in given case of highly pure materials.
For fine classification of solid materials cyclones, hydraulic and dispersive bowl classifiers, spiral air elutriators and centrifuges are used.
The mathematical definition of the flow taking place in cyclones has been unsuccessful so far. The lifting and extracting forces applied to the grains in the flow tube (there is only one in the cyclone) are not constant in the cyclone, hence they are unsuitable for sharp classification. A further disturbing effect is that, due to the shape of the cyclones, the flow-tube does not fill out the full cross section along the horizontal and vertical (intersecting) planes, thus disturbing convection flows develop, further deteriorating the classification capacity. As a result, the cyclones are mainly used, for dust separation, or sludge thickening, instead of classification. However, the cyclones do not function perfectly for dust separation either, because not even the constant intensification of the extracting force towards the center is ensured by run of the flow line.
According to the DE-PS 2 536 360 a cyclone is used for the supply of accelerating air to separate the solid particles of the gaseous medium. In the DE-PS 2 942 099, the separation adjusting nozzle at the outlet of the hydro-cyclone used for sand fractionation is formed to be elliptical to improve the classification.
In case of cyclones used for dust separation (see DE-PS 2 826 808) several holes are arranged on the bottom of the separating chamber between the dust-tube and the storage tank for exhaust of the dust-air mixture.
In the hydraulic and dispersive bowl classifiers laminar upward flow of constant velocity is in a tube or tank, in which only the grains of higher falling velocity than a given limit are capable to fall down upon the effect of gravitation, to be removed by a discharge mechanism from the bottom of the vessel. The fine grains together with the flowing medium leave through the overflow lip of the vessel.
In case of hydraulic classifiers the medium is pressed into the vessel by one or possibly several external pumps. In the apparatuses functioning with gaseous medium, the fan wheel bringing about air circulation is arranged within the classifier on its upper part, generally on the same shaft with the dispersive bowl, the purpose of which is the uniform dispersion of the material in the upward flowing medium. A drawback of the apparatus is that it functions in relatively coarse grain size range, because very low falling velocities are given in the gravitational field, e.g. for the grains smaller than 20 .mu.m. The sharpness of the classification is not satisfactory either, because the laminar flow cannot be provided for. With hydraulic apparatuses the medium entering through the small cross section ought to be distributed at a uniform rate generally in very large cross section, which is an insoluble problem. While in the apparatuses functioning with gaseous medium, the rotation of the fan wheel produces turbulence. Owing to the inadequately sharp classification, the hydraulic classifiers are generally used as auxiliary aid in mineral preparatory processes, while these types of the classifier are used only where no sharp classification is required, e.g. as intermediate classifier in grinding cycle.
The efficiency of the centrifugal classifiers is poor. Namely, in the centrifuge, extracting force is applied to each grain towards the outer wall of the vessel (to increasing extent). Hence the centrifuges (drum, worm, sieve-types, etc.) are very good for sludge thickening, or dewatering, but as classifiers they function with poor efficiency. The classification is made possible only by the medium flowing in the centrifuge drum perpendicularly to the falling direction of the grain, and the very fine grains not yet settled until the overflow are capable to emerge together with the liquid. This, however, represents a relatively wide range and not a specific size.
Such apparatuses are described in the DE-PS 2 556 382 and 2 649 382.
The spiral classifiers are the presently known sharpest classifiers.
The DE-PS 2 629 745 discloses an approximate mathematical model of the flow. The shape and velocity of the flow tube and the acceleration ratios are such that lifting, extracting forces of the same extent are applied to the grains. Thus these classifiers separate more or less at a specific grain size. Their drawback is partly that the suitable run of the flow line can be accomplished only with the fast rotation of the classifying chamber walls (flat cylindrical space), and partly that as a result of the law of continuity only one side of the space would be confined by a flat surface. Disregarding this aspect results in reduced sharpness of the classification. On the other hand, the presence of rotary parts mechanically (statically) limits the grain size range, in which the classifier is capable to function. Namely, the separated grain size can be controlled by varying the vane angle on the circumference and the rotational velocity of the chamber-wall, which influence the shape of the flow-tube. The output of the machine is limited by the chamber-wall and exhaust fan being mounted on a common shaft, consequently the amount of exhausted air is also limited.
A version of the former classifier is the system where the run of the spirals is controlled by the rotational velocity of the central rotary part provided with radial slots, instead of changing the vane angle. The main drawback of both systems is that the rotary parts wear off at a fast rate upon the effect of the hard grains, consequently they can be used only for the classification of soft materials.